Field of the Invention
The present invention relates to extending and enhancing bright coherent high-order harmonic generation into the VUV-EUV-X-ray regions of the spectrum. In particular, the present invention relates to phase matched and effectively phase matched generation of VUV, EUV, and X-ray light using VUV-UV-VIS laser pulses.
Discussion of Related Art
This invention is related to three previously demonstrated techniques, implemented in a novel regime of parameters that contrasts with the parameters of all techniques demonstrated to date:
The first related technique is developed by some of the inventors in the current application and includes a method and device for phase matching the generation of high harmonic radiation in a waveguide, which is patented (U.S. Pat. No. 6,151,155, incorporated herein by reference). Using this technique and, typically, a Ti:Sapphire laser of a wavelength of 0.8 μm to drive the high harmonic process, full phase matching of the process can extend to EUV—soft x-ray wavelengths of about 10 nm. Therefore, in practice, the output flux produced is significant enough to enable applications using light with wavelength no shorter than 10 nm. In this scheme, the generation of soft x-rays of shorter wavelengths requires an increase in driving laser intensity. However, scaling of macroscopic parameters (see typical values in Table 1, Regime I) does not allow for extending the phase matching of the process to shorter x-ray wavelengths, at the same time. Higher intensities result in too much ionization of the gas, degrading the phase matching conditions and limiting both the flux and the coherence of the generated light. Emission at wavelengths shorter than ˜10 nm can be observed but it originates from a region of the nonlinear medium of short length, and.or with a low density of emitters, and in the presence of a large number of free electrons All these factors dramatically decreasing the x-ray flux to levels not useful for applications. Typically, the emission originates from a short coherence length of <1-100 μm and both atoms and ions can emit harmonics of very high photon energies. However, due to the very short coherence length—that can be much shorter than the medium absorption length and medium length—the emission from ions in the high photon energy range is 3-4 orders of magnitude lower than the emission from atoms limited only within the low photon energy range.
The second technique related to this invention is the idea that a longer-wavelength driving laser can be used to generate X-rays of shorter wavelength. This idea originates from the simple theoretical model that well describes the high-order harmonic generation process (Kulander; Corkum). However, subsequent theory predicted that using longer laser wavelengths λLaser to generate shorter X-ray wavelengths comes at a cost of a significantly lower single-atom yield (i.e. the amount of X-ray light generated for each atom in the nonlinear medium, predicted to scales as ˜λLaser−3). Initial attempts to generate shorter-wavelength X-rays using laser wavelengths as long as 1.5 μm (Shan, PRA 65, 011804 (2001)) supported this prediction of diminishing flux. This and later experiments were done in a gas jet geometry using macroscopic parameter regime similar to the well known Regime I (a low gas pressure (on the order of tens to hundreds of torr) in the interaction region, and a short propagation length (on the order of 0.1-1 mm)), and did not observe flux that would make their approach usable as a general purpose coherent X-ray source. Furthermore, more recent quantum calculations suggested that the single-atom yield drops even more dramatically with laser wavelengths and scales as λLaser−5.5. The ideas in this approach imply that shorter-wavelength lasers may give rise to significantly bright emission from a single emitter limited to the long-wavelength HHG range, however, the macroscopic optimization of the HHG upconversion has not been considered neither for shorter-wavelength nor for longer-wavelength lasers.
To remedy the low conversion efficiency observed in the above-mentioned work, in past work we developed a third technique—a method for optimization of phase matched conversion of mid-infrared laser light into the X-ray region of the spectrum. By using high-pressure medium and a loose-focusing geometry. This approach, devised by many of the same inventors as the current invention, is described is described in Popmintchev et al., U.S. Pat. No. 8,462,824 (incorporated herein by reference). In contrast with the previously discussed scheme in Regime I, the inventors demonstrated that both the X-ray radiation from a single atom, as well as phase matched conversion of this process, can be extended to the generation of much shorter wavelengths. In order to produce significant X-ray flux, this techniques requires a regime of parameters (see optimal parameters in Table 1, Regime II), contrasting with the parameters in the scheme illustrated in Regime I using Ti:Sapphire lasers. This phase matching scaling to shorter wavelengths is achieved through increasing the laser wavelength while slightly decreasing the laser intensity. Second, a nonlinear medium of much higher density (note the optimal gas pressure of multiple atmospheres) is required when using longer wavelength lasers, to allow the X-ray beam intensity to build to optimum conversion efficiency. This technique employs nonlinear media that is only barely ionized, with many neutral atoms still remaining after the generation process terminates. Carefully designed macroscopic parameters lead to coherent build of the X-ray signal over extended optimal medium length that can be a significantly long (cm or longer). The result is a fully spatially and temporally coherent, well directed, X-ray beam. Tunability of this broadband X-ray source is achieved by changing the laser wavelength in the IR region and by changing the intensity and/or the medium, plus adjusting the medium parameters. The tunability range can be from hundreds of nm to sub-nm. Helium gas is a good candidate as a nonlinear medium usable for a tunable source over a broad range of X-ray wavelengths. Other noble gases can also be used in some regions of the spectrum. Finally, the method for optimization in Regime II implies that using shorter-wavelength laser can generate bright phase-matched HHG from partially ionized atoms in the long-wavelength VUV range.
A need remains in the art for using shorter wavelength driving lasers in the VUV-UV-VIS region of the spectrum to benefit from the strong single-atom yield, while simultaneously extending this upconversion in an efficient way well into the shorter wavelength VUV, EUV, and X-ray regions.